Enhancement: Improve error reporting when image pull job expires

[senthilrch]

When images cannot be pulled into cache due to issues like incorrect image path/tag, incorrect image pull secrets etc. the image pull job expires after --image-pull-deadline-duration. The image cache status is updated with empty failure message and reason. A suitable error message and reason would help users to understand the cause of the failure. See below:-

},
"status": {
"completionTime": "2021-08-09T21:26:09Z",
"failures": {
"myreg.azurecr.io/private-a": [
{
"message": "",
"node": "aks-m-20868404-vmss00006h",
"reason": ""
},
{
"message": "",
"node": "aks-m-20868404-vmss00006j",
"reason": ""
},
{
"message": "",
"node": "aks-m-20868404-vmss000004",
"reason": ""
},
{
"message": "",
"node": "aks-m-20868404-vmss000065",
"reason": ""
},........